Numerous variants of membrane filtration units are known.
By way of example, EP-B-0 181 470 has disclosed a membrane filtration unit with a case-like housing in which there is a bundle of 5,000-10,000 hollow membrane fibres. End parts of the membrane fibres are embedded in resin-like sealing bodies which separate the housing into separate spaces, one which is in contact with the outer side and one which is in contact with the inner side of the membrane fibres.
A drawback of this known membrane filtration unit is that there is a considerable risk of the membrane fibres breaking during operation. The transitions from the embedded parts of the membrane fibres to those parts of the membrane fibres which are not embedded and extend between the embedded parts are particularly fragile. This imposes limits on process conditions (such as feedstock and backflush flow rates, maximum operating temperature and feedstock and backflush pressures) under which the unit can be used.
U.S. Pat. No. 3,832,830 shows a membrane filtration unit having a housing in which there is a bundle of more than a million hollow membrane fibres. Once again, end parts of the membrane fibres are embedded in resin-like sealing bodies. The bundle of membrane fibres is enclosed by a flexible, porous, elongate piece of tubing which extends over the entire length of the bundle and is likewise embedded in the resin-like sealing bodies. The flexible piece of tubing holds the bundle of membrane fibres together and imparts a certain rigidity and strength to these fibres. The piece of tubing facilitates positioning the enormous number of membrane fibres in the housing without the membrane fibres in the bundle being pressed flat or damaged. To reduce the risk of breakage at the location of the fragile transitional parts between the embedded and unembedded parts of the membrane fibres, the unit comprises elastic retaining elements which are arranged around the fragile transitional parts and part of which extend into the sealing bodies, to which they are fixedly connected by being embedded. At the location of the transitional parts, the elastic retaining elements wrap taut around the bundle of membrane fibres and the piece of tubing surrounding them, serving primarily to prevent resin being able to penetrate into the unembedded part of the bundle during production of the sealing bodies. As a result, the tightly wrapped transitional parts become inflexible.
A drawback of this known membrane filtration unit is that in practice it has been found that not only does the bundle of tightly wrapped transitional parts of the membrane fibres become inflexible, but a large part of the flexibility of the part of the bundle which is enclosed only by the piece of tubing is also lost. In addition, there are, as it were, new fragile transitional parts of the membrane fibres formed in the vicinity of the unembedded ends of the retaining elements, where there is a relatively high risk of the membrane fibres breaking. Furthermore, this membrane filtration unit is expensive and complex to produce.
JP-A-11179164 discloses a membrane module comprising a cylindrical housing in which a bundle of hollow fibre membranes is housed. The bundle of membranes is inserted into a water-permeable cover cylinder. The housing comprises end parts having a larger diameter, where resin partition walls are provided inside which the ends of the membranes are embedded. Both ends of the water-permeable cover cylinder are not buried in the resin partition walls.